This invention relates to a radio receiver having an automatic gain control (AGC) circuit, and more particularly relates to such a radio additionally having an AGC signal restorer circuit having an output linearly proportional to the signal strength of the incoming radio signal.
It is well known to employ an AGC circuit in a radio receiver to automatically adjust the gain in one or more of the amplifier stages so as to keep the radio output signal at about the same level while the amplitude of the incoming radio signal may vary several orders of magnitude. Thus the convenience and pleasure of the listener is enhanced while the signal levels in the stages under AGC influence (in the loop) is kept to within a linear range avoiding distortion such as that due to overloading or "saturating", or from another view, making the construction of such stages simpler and less costly.
In an AM, superheterodyne receiver, such as that illustrated in FIG. 1, the DC component of the output signal from a simple rectifier-type AM detector is proportional to the amplitude of the IF carrier at the output of the IF amplifiers. That DC component is typically transformed into a suitable signal for controlling the gain of at least one or more IF stages and the AGC feedback loop includes the IF stage(s), the detector and the AGC circuit.
Thus the AGC circuit insures that the range of signal amplitudes that need to be handled without distortion in the IF amplifier, detector and audio amplifier, is quite small and consequently the design of these circuits is made less critical and more economical.
When the radio includes a signal strength meter, the meter is usually driven by the AGC circuit. However, such an arrangement usually leads to quite a nonlinear meter response with respect to the amplitude of the IF input signal because, owing to nonlinearities in the AGC circuit that is relatively insensitive at moderate to large signal levels, the DC level at the input of the AGC circuit has a nonlinear relationship with the magnitude of the incoming IF signal as illustrated by curve 11 in FIG. 2. When a more precise measure of incoming signal strength is wanted, a DC signal having a linear relationship with the incoming signal would be desirable.
Of course one could derive such a desired signal by simply duplicating those parts of the radio, (e.g. IF, detector and audio amplifier) but without an AGC. However, this would lead to high costs especially since without AGC the range of amplitudes that those stages must handle is large and their design would be more critical and complex.
When the radio employs an automatic scan-tuning means for scanning and stopping at a strong station, the AGC signal typically serves also as the scan-stop signal. By their very nature AGC siganls of AM radios incorporate substantial delay, overshoot and/or undershoot in response to a fast change in amplitude of the radio signal. Thus the tune-scanning rate must be kept low in order to avoid lock-up on strong stations and skipping moderately strong stations. Such scanners typically take no less than 15 seconds to scan the AM broadcast band, which is much slower than desired. In general the speed of response of the AGC circuit in an AM receiver is no greater than can be developed from sub-audible frequencies whereas by contrast, the response of the AGC circuit in an FM receiver is limited only by the band width of the IF amplifier.
Therefore, an object of the present invention is to provide in a radio receiver capable of operation in an analogue gain mode, such as an AM radio receiver, an economical means for producing a monitoring signal that is proportional to the amplitude of an incoming radio signal.
It is a further object of this invention to provide in a scan-tuning AM radio, an economical means for providing a stop signal to the scan-tuning circuit that does not include the delays, overshoot, undershoot and nonlinearities of the AGC circuit.